The present invention relates to double sheet detection method and apparatus of a sheet-fed rotary press.
In conventional sheet-fed rotary presses, when two sheets are simultaneously fed to a feedboard, they are detected through a through hole of the feedboard by a photodetector consisting of a light source and a photosensor so as to stop the operation of the press. Conventional detecting methods are shown in FIGS. 1 to 3, respectively.
FIGS. 1 to 3 are graphs each of which shows the relationship between a transmittance .alpha. of light through a sheet and an amount D of light transmitted therethrough. It should be noted that the amount of light is expressed as a percentage where an amount of light which corresponds to 100% of the transmittance is given to be 100%. The transmittance .alpha. and the amount D have a linear relationship (D=.alpha.) when one sheet is subjected to detection, as indicated by a line A. However, the transmittance .alpha. and the amount D have a nonlinear relationship (D=.alpha..sup.2) when two sheets are subjected to detection. When the sheets have the same quality and thickness, the amount of light transmitted through one sheet is greater than that transmitted through two sheets. Double sheet detection is performed in accordance with a difference between these amounts.
According to the method shown in FIG. 1, a detection level Ld is fixed in accordance with the types (thickness and quality) of sheet. In other words, each detection level is given for the corresponding type of sheet. A detectable range DE of this method is very narrow, and the detection level must be reset in accordance with each different type of sheet. In addition to these disadvantages, changes in various conditions over time cannot be compensated for by this method.
In the method shown in FIG. 2, the previous amounts of light transmitted through the given type of sheets are averaged. Data representing an average amount of light is stored in a memory, and a detection level Ld is determined in accordance with this data. In comparison with the method shown in FIG. 1, a detectable range DE of the second method can be greatly increased. However, when a transmittance is close to 0% or 100%, double sheet detection cannot be performed.
In the method shown in FIG. 3, a detection level Ld is determined by multiplying a given ratio with the data stored in the second method. A detectable range DE of the third method is wider than that of the second method. However, when a transmittance is close to 100%, double sheet detection cannot be performed.
In order to solve the above problems, still another conventional method is proposed in Japanese Patent Disclosure No. 57-214994 entitled "Double sheet detection method and apparatus for sheet-fed rotary press" of the same applicant as that of the present invention, wherein double sheet detection can be performed in accordance with an optical reference value which can be automatically set even if a transmittance substantially varies from 0% to 100%. This method comprises the steps of: setting a theoretical reference value as an intermediate value between a first theoretical amount of light transmitted through one sheet and a second theoretical amount of light transmitted through two sheets, respectively; subtracting the theoretical reference value from the first amount of light to obtain a theoretical subtracted value; subtracting the theoretical subtracted value from an actual amount of light transmitted through one sheet to obtain an actual reference value; and comparing an actual amount of light transmitted through a current sheet with the actual reference value to perform double sheet detection. According to this method, the actual reference value and the amount of light transmitted through the sheet are considerably large values. In this sense, a comparator must have a wide input range, resulting in high cost.